A known Frequency Lock Loop (FLL) of a passive atomic frequency standard is shown in FIG. 1. It comprises a Voltage Controlled Oscillator (1), usually a Crystal Oscillator, which outputs an RF signal at 10 MHz. The RF signal is modulated by a modulator (7) at low frequency, and is multiplied and synthesized by a multiplier/synthesizer (2) which multiplies and synthesizes the said RF signal to the Microwave range, to produce a signal at 6.84 Hz (for a Rubidium Standard) or of 9.26 Hz for a Cesium Standard. The said Microwave signal is injected into an Atomic Resonator (3) which may be a Rubidium Physics Package working on a principle of an optical pumping, or a Cesium Beam Tube.
The Atomic Resonator (3) outputs an error signal which is being amplified by the Preamplifier (4) followed by a demodulator (5) which demodulates the error signal and an Integrator (6) which integrates the signal and controls the VCO. This is a common FLL scheme for Atomic Clocks (Ref. 1).